Optical absorption spectrometers are commonly used for gas phase absorption spectroscopy using the infrared, visible and ultraviolet parts of the optical spectrum (referred to collectively herein as “light”).
The detection sensitivity of an optical absorption spectrometer depends on the interaction length, described by the de-Beer law.
The de-Beer law states:P=P0 exp(−σNI)
Where P is the transmitted optical power, P0 is the incident optical power, σ is the absorption cross section of the gas in m2 per molecule (a characteristic of the gas), N is the gas concentration in molecules per m3 and I is the interaction length (the length of gas that interacts with the light). This demonstrates that the absorption signal depends on the length of the gas column through which the light passes.
In order to maximise the interaction length in a cell of practical dimensions, there are a number of multiple pass optical designs which are based on sequential imaging of the optical beam using curved mirrors. The first multipass cell design was originally proposed and demonstrated by White (White, 1942) and is referred to generically as a White cell. This design is however rather sensitive to the optical alignment of the mirrors, typically measured in tens of microradians.
Later improvements, of relevance to infrared spectrometers, were introduced by White in which additional prisms were incorporated to increase the number of passes while reducing the mirror alignment sensitivity (White, ˜1970's).
Alternative cell designs have been described by Herriott with the aim of producing a very high number of optical passes in the cell for use in infrared spectroscopy.
In a multipass cell the de-Beer law is modified as below:P=P0 exp(−σNKL)Where L is the distance between the mirrors, and K is the number of optical passes.
Multipass cells as described above typically have a distance L in the range 400 mm to 1000 mm, and the number of optical passes K within the cell is typically in the range 12 to 500 passes. Thus the effective length over which the gas interacts with the light is typically in the range 4 meters to 500 meters.
The optimum number of passes K is a function of the optical losses in the cell. This is primarily related to the imperfect reflectivity of the mirrors. Multipass cells which operate in the infrared use very low loss mirrors having a reflectivity of about 99%, which allow a large number of passes. However, mirrors designed for use in the visible and ultraviolet (UV) regions of the spectrum have a lower reflectivity and hence much higher reflection losses, and this reduces the optimum number of passes. This is particularly a problem with UV mirrors, which typically have a reflection loss figure of greater than 10%, and it is also a smaller but significant problem with visible light mirrors.
Thus, multipass cells which operate in the UV (typically 150 nm to 400 nm) and visible (typically 400 nm to 700 nm) regions of the optical spectrum typically operate with 12 to 50 optical passes.
To summarise:
SpectralOptimum NumberregionMirrorsof passesLamp usedUVEnhanced aluminium12 to 40DeuteriumVisibleEnhanced aluminium 28 to 50Tungstenor dielectricIRGold 50 to 1000Diode laser
Typically multipass optical cells are incorporated into a measurement instrument. Additional subsystems are necessary to make such an instrument work satisfactorily. A suitable light source with associated collimation optical system is necessary to illuminate the multipass optical cell. The light source is preferably a continuous broadband light source, typically for example a Xenon arc lamp. Light exiting the cell is focused on the entrance slit of a spectrometer to provide an electrical signal characteristic of the fingerprint absorption spectrum of the gasses in the cell. An electronics subsystem running suitable algorithms carries out analysis of the spectrum in order to provide a measurement of the concentrations of gases in the cell.
Having arrived at the optimum number of passes, the only way in which the sensitivity of the instrument can be increased is to increase the cell length.
However, this gives rise to a design conflict. Since an optical cell is a simple structural element, the bending stiffness is proportional to the 1/(cell length)3. Thus, for example, doubling the cell length to improve the detection sensitivity by a factor of two reduces the cell stiffness by a factor of eight, to the detriment of the long term stability of the instrument.
Furthermore, the longer the cell and/or the higher the optimum number of passes, the more difficult it is to align the optics. Generally doubling the cell length halves the allowable mirror misalignment. Likewise doubling the number of passes halves the allowable mirror alignment.
It would therefore be desirable to be able to increase the path length without increasing the cell length. However, in a conventional multipass cell this cannot be achieved without increasing the number of passes, which is prevented by the imperfect reflectivity of the mirrors particularly when operating in the infrared and also to a lesser degree when using visible light.
It is an aim of this invention to mitigate one or more the above problems.
U.S. Pat. No. 5,943,136 describes an optical cavity resonator device that is designed for measuring optical absorption using a high-Q optical resonant cavity. The device uses total internal reflection to generate an evanescent wave that decays exponentially at a point external to the cavity. Absorbing materials placed outside the cavity in the vicinity of this evanescent wave alter the Q-factor of the cavity, thus allowing the material to be probed. The device operates in entirely different way to the multipass spectroscopic absorption cell described herein, as the sample gas is not contained within the optical cavity. The patent is therefore mentioned here only for background interest.